A multi-cylinder internal combustion engine comprises a number of cylinders, each of which cyclically burns a mixture that is composed of a comburent (fresh air taken in from the atmosphere) and a fuel (petrol, diesel fuel or similar) and which must have mixture ratio values (i.e. the ratio between comburent and fuel) equal to an intended value that is variable depending on the engine running condition and is generally close to the stoichiometric value necessary for the correct functioning of the catalytic converters in the exhaust system.
In order to optimize the conversion efficiency of the catalytic converter, it has been proposed to make the mixture ratio value (and therefore the oxygen content in the exhaust gas) oscillate around a mean value equal or close to the stoichiometric value by using a sinusoidal pulse having amplitude and frequency dependent on the physical characteristics and age of the actual catalytic converter.
Measurements of the oxygen content of the exhaust gas, which is provided by a lambda sensor positioned upstream of the catalytic converter, are used to control the mixture ratio.
When a single lambda sensor is placed upstream of the catalytic converter, the measurement provided by the single lambda sensor is used to control the mixture ratio of all the cylinders in the internal combustion engine. In particular, a single PID controller, which regulates the amount of fuel injected, is used to track an intended value for the mixture ratio, using the measurement provided by the single lambda sensor as a feedback variable.
When several lambda sensors are present, the cylinders of the lambda sensor equipped engine are divided into a number of groups (normally composed of one to three cylinders) and each lambda sensor is installed upstream of an exhaust manifold that merges the exhaust gas of all the cylinders in a manner such that the same lambda sensor measures the oxygen content of the exhaust gas of a respective group of cylinders; the mixture ratio of each group of cylinders is independently controlled from the mixture ratio of the other groups of cylinders by using the measurement provided by the respective lambda sensor. In particular, a PID controller is used for each respective group of cylinders, which regulates the amount of fuel injected into the group of cylinders to track an intended value for the mixture ratio by using the measurement provided by the respective lambda sensor as a feedback variable.
The above-described way of controlling the mixture ratio presents some drawbacks when several lambda sensors are present, as it is difficult to achieve the intended oscillation in the mixture ratio of the exhaust gas fed to the catalytic converter as the mixture ratio controls of the various groups of cylinders are mutually independent. In other words, each mixture ratio control tries to achieve the intended oscillation in the exhaust gas mixture ratio, but the oscillations caused by the various mixture ratio controls might not be perfectly timed due the inevitable presence of small asymmetries and therefore the overall oscillation (constituted by the sum of the oscillations caused by the various mixture ratio controls) that affects the catalytic converter might be very different from the intended oscillation, both in terms of amplitude and frequency.